1. Field of the Invention
The present invention relates to a diaphragm control apparatus for camera in which the diaphragm is automatically controlled while monitoring the aperture value relying upon the output from a light metering circuit which measures object light passed through the photographing lens and the diaphragm aperture. More particularly, the present invention relates to a diaphragm control apparatus for camera of the type in which the light passed through the diaphragm aperture is continuously measured while the diaphragm being stopped down gradually from its full open state prior to start of exposure to continuously detect the photometric output changing with time and, when the photometric output has reached a certain determined value, the aperture stopping down motion is blocked. This type of light metering system is generally called instantaneous stop-down metering system.
2. Description of the Prior Art
In the camera art there is known such diaphragm control apparatus which functions as a shutter speed priority automatic exposure control and which sets the diaphragm aperture to a proper value by blocking the aperture stopping-down motion of the diaphragm when the photometric output according to the instantaneous stop-down metering system has reached a certain determined value. This type of known diaphragm control apparatus uses an electromagnetic mechanism to block the stopping-down motion of the diaphragm. Typically, the electromagnetic mechanism comprises essentially an armature serving as a blocking pawl and an electromagnet for driving the armature. The electromagnet is actuated by a stop-down blocking signal which is issued when the photometric output has reached a proper value. As will be readily understood, there occurs some time lag between the time point of signal input and that of actual stoppage of the aperture stopping-down motion. This results in overshooting of aperture stopping-down to an extent corresponding to the time delay. Namely, error is produced in stopping down the aperture. This stop-down error is an important problem involved in the above mentioned type of known diaphragm control apparatus.
To solve the problem it has been proposed to provide a differentiation circuit in the diaphragm control apparatus. The differentiation circuit differentiates the stopping-down photometric output time to know the value of component of the metered output corresponding to the stop-down error from the differentiated output. The thus found value is used to correct the stop-down error. Representative techniques to correct the stop-down error using a differentiation circuit are disclosed in U.S. Pat. No. 3,987,460 which corresponds to German Pat. No. 2,530,544 and U.S. Pat. No. 4,179,202 which corresponds to German Pat. No. 2,842,339.
These prior art techniques disclosed in the abovementioned patent specifications will be described in detail hereinafter.
According to the technique disclosed in the first mentioned patent publication a photometric output which decreases with stop-down time is linearly differentiated to obtain an output corresponding to the decreasing rate of the photometric output. The output thus obtained is multiplied by a delay time as mentioned above to obtain a signal corresponding to the stop-down error. This error signal is added to the photometric output as a correction value so that a stop-down blocking signal is issued a little early. Since the blocking signal is issued a little early and the stopping-down motion is actually blocked after the lapse of the delay time, there is obtained an aperture value which is approximately proper when the stopping-down motion is blocked in effect. As will be seen from the above, this prior art technique determines the correction value assuming that the decrease of the stop-down metered output is a uniform motion. However, in practice, such cases are very few in which the decrease of photometric output may be considered approximately to be a uniform motion. Therefore, error was still produced in stopping down the aperture even when a correction was made according to this technique.
The second technique disclosed in the latter mentioned patent publication is a development of the above prior art technique. The patent has proposed to add further a second correction value to the correction value used in the former. The second correction value is obtained by a quadratic differentiation of the stop-down metered output and a multiplication of the value resulted from the differentiation, that is, the decreasing acceleration of the metered output by the square of delay time. Thus, according to the latter technique, the correction value is determined assuming that the decrease of the metered output is a uniformly accelerated motion. It is true that the accuracy of timing for blocking the stop-down motion was improved by this technique. However, there is still such case in which the photometering output/stop-down time characteristic curve is very complicated so that it can not be approximated to a uniformly accelerated motion. In this case it is impossible to correct stop-down error even when the latter-mentioned technique is used.
In both of above described prior art techniques, differentiation circuit is used to differentiate the photometric output, which brings forth the following disadvantages common to both the techniques:
Generally, the photometric output is obtained by amplifying a weak output from a photo-electric element. At this step of amplification, the photometric output often varies tremulously because of harmonic noise contained in the output. Apart from this cause, the photometric output is variable tremulously also under a light source flickering at high frequency such as a fluorescent lamp. When a photometric output containing such variable component is differentiated, the differentiated output of the variable component is enhanced to a great extent relative the intrinsic photometric output and becomes illegibly large. Since the second mentioned prior art technique uses a quadrically differentiated output, this disadvantage is much more remarkable in the second prior art technique rather than in the first one. It is obvious that when such a differentiated output is added to the photometric output as a correction value, stable and precise blocking of the aperture stop-down is no longer attainable.